Stimulated Brillouin scattering for fiber-optic links

ABSTRACT

An attenuator/filter is disclosed that is inherently tuned and provides for a stimulated Brillouin scattering effect having a predetermined threshold that fixes the output thereof at a predefined power level. The stimulated Brillouin scattering effect is disclosed as being provided by either a fused silica optical fiber or a fiber-optic ring resonator both of which act as a filter to allow selected frequencies to pass. The SBS threshold is selected to improve the sideband to carrier power ratio which, in turn, determines the modulation depth for the modulated signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to sensing, transmitting and processing ofelectrical signals carried by an optical carrier wave. Moreparticularly, the present invention relates to modulating opticalcarriers, having sidebands, with relatively weak signals, and improvingthe power ratio of the sidebands relative to their carrier wave by theuse of a Brillouin medium.

2. Description of the Prior Art

The processing systems that utilize fiber optics and electro-opticalmodulators to modulate information carried by optical carrier waves andtheir sidebands are of ever increasing importance. The modulationefficiency of electro-optical modulators has a direct impact on theefficiency of the optical processing systems. One of the contributingfactors that degrades the modulation efficiency is that most modulatorsexhibit large switching (on-off) voltages and so small input signalshave little effect on the optical carrier transmitted from theelectro-optical modulator. The modulator output commonly consists of astrong unmodulated optical carrier with relatively weak signalsidebands.

The strong unmodulated optical carrier is further typically andundesirably strengthened because it is desired to have high opticalpower levels in order to create relatively high signal-to-noise ratiosof the signals being carried by the optical carrier. However, thesestrong unmodulated optical signals that correspond to weakly modulatedoptical waves (having small modulation indices, commonly referred to asa depth of modulation) leave significant unmodulated signal power in theoriginal carrier. Hence, despite a high average optical power needed toincrease the electrical signal power for desired signal-to-noise ratios,the electrical power carried by the modulated signals may be quitesmall. The unmodulated signal, that is, excess optical power not beingutilized for modulation purposes is detrimental in at least two ways.First, optical amplifiers are limited by both their average input andoutput powers; therefore, optical amplifiers will have limited use toboost the weakly modulated optical wave contained in the carrier signal.More particularly, the high unmodulated signal sets the operating valueof the amplifier which may not allow for the amplification of the lowmodulated signal having a need for amplification. Second, and even moreimportant, the average optical power must be kept below approximately 5milliwatts (mW) to avoid signal reduction, distortion and damage to thephotodetector that commonly receives the output of the electro-opticalmodulator. This 5 mW limitation does not take advantage of the existingoptical signal generator having average optical power outputsapproaching 100 to 200 mW levels.

Accordingly, it is desired that an apparatus and a method of operationthereof be provided to reduce the unmodulated carrier power so thatamplifiers can be efficiently used and so that optical carriergenerators having relatively high output power levels may be detected bya photodetector without any operation degradation of the photodetectoror without any damage to the photodetector. If such an apparatus andmethod are provided then optical amplifiers may be more efficientlyutilized and a corresponding reduction in the fiber-optical link lossmay be realized. Furthermore, the provided apparatus and method may beutilized in conjunction with other optical systems that utilize linearcarrier filtering techniques so as to make additional improvements inthe fiber-optic link efficiency. If such an apparatus and method ofoperation thereof are provided the sensitivity and efficiency of theoverall fiber-optical link or its related system may be improved.

OBJECTS OF THE INVENTION

Accordingly, a principal object of the present invention is to provideboth an apparatus and a method for increasing the sensitivity andefficiency of the fiber-optical transmission and processing systems.

Another object of the present invention is to reduce the level of theoutput power of an electro-optical modulator forming part of thefiber-optical system so that the output power signal does not distort orreduce the output of the photodetector receiving the optical modulatedsignal, nor does the output power signal damage such a photodetector.

It is a further object of the present invention to provide for opticalcarrier wave sources having outputs in excess of 5 milliwatts, yet allowhigh speed photodetectors to receive and respond to signals of about 1to 5 milliwatts (mW) so as to avoid catastrophic damage to thephotodetector receiving a modulated signal, while at the same timeproviding for a linear response of the photodetector.

Further, it is an object of the present invention to provide for anapparatus and a method of modulating optical signals, such as thosedeveloped by lasers and allow for a very broadband response of suchmodulators in forms of tens of megahertz to tens of gigahertz, so thatthe practice of the present invention may easily find application inwideband systems using optical transmission and receiving techniques.

SUMMARY OF THE INVENTION

The present invention is directed to optical systems utilizing aBrillouin medium that has a fixed operation at a definite power level sothat signals, such as optical carrier waves, above this level areattenuated, while signals below this level, such as those contained inthe sidebands of the optical carrier waves, are allowed to pass.

The present invention provides an attenuator and filter for reducing,passing, and blocking incident optical signals comprising carrier waveshaving sidebands each having a preselected power level. The attenuatorand filter comprise a Brillouin medium having a threshold for creatingbackward traveling Stoke's waves when subjected to a predetermined powerlevel of incident optical signals. The threshold is selected to increasethe power ratio of the sidebands to their carrier wave.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the invention, aswell as the invention itself, will become better understood by referenceto the following detailed description when considered in connection withthe accompanying drawings, wherein like reference number designateidentical or corresponding parts throughout the several views andwherein:

FIG. 1 is a block diagram of prior art arrangement having aphotodetector at the output stage thereof responsive to optical signals.

FIG. 2 is a diagram illustrating the quadrature biasing and outputparameter of the electro-optical modulator shown in FIG. 1.

FIG. 3 is a schematic of the components making up the output powersignal of the electro-optical modulator of FIG. 1.

FIG. 4 is a block diagram illustrating the general principles related toa stimulated Brillouin scattering device utilized in the presentinvention.

FIG. 5 illustrates a schematic related to the generation of the Stoke'swaves of the Brillouin device generally illustrated in FIG. 4.

FIG. 6 illustrates the response of the Brillouin device of FIG. 4.

FIG. 7 illustrates the response of the circuit arrangement of FIG. 4.

FIG. 8 illustrates an alternate embodiment for generating the Stoke'swaves of FIG. 5.

FIG. 9 illustrates a block diagram of a system utilizing the practice ofthe present invention.

FIG. 10 illustrates various responses of the system of FIG. 9 tocorresponding stimulation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, the present invention relates to the use of a Brillouinmedium, as a filter and attenuator, that creates Stoke's waves whensubjected to an optical carrier having sidebands, each with apredetermined power ratio. The Brillouin filter and attenuator has athreshold, commonly referred to as a stimulated Brillouin scattering(SBS) threshold, that fixes the operation thereof to a predeterminedpower level output so that selected signals are attenuated and selectedsignals are allowed to pass through the filter and attenuator withoutany attenuation thereto. The principles of the present invention may bebetter appreciated by first referring to a prior art arrangement 10illustrating in FIG. 1.

The prior art arrangement 10 comprises a carrier wave (CW) source 12,and electro-optical modulator 14, an optical fiber 16, and aphotodetector (PD) 18. The CW source 12 may have a power output of aboutfive to twenty milliwatts (mW) which is relatively low considering thatpresently existing laser sources may have power levels approaching 200mW which cannot be used, in a manner to be described, for thearrangement 10 having the photodetector 18. The carrier wave (CW) source12 generates an output signal indicated in FIG. 1 as P_(in) and appliedon signal path 20 that is routed to the electro-optical modulator 14.

The electro-optical modulator 14 may be a Mach-Zehnder modulator whichdepends sinusoidally on an input electrical signal, such as signal 22(V.sub.π Input), that is applied to signal path 24 which, in turn, isrouted to the electro-optical modulator 14. The electro-opticalmodulator 14 modulates the carrier wave with the information containedin signal 22, thereby, allowing the carrier wave having sidebands toserve as a vehicle for optically transmitting the information containedin signal 22. The electro-optical modulator 14 generates an outputsignal, indicated in FIG. 1 as P_(out), that is applied to signal path26 which is routed to the fiber optic 16 which, in turn, applies anoutput signal on signal path 28 that is routed to the photodetector (PD)18 which, in turn, provides an output signal 30 (OUT) on signal path 32.The carrier wave (CW) source 12, the electro-optical modulator 14, fiberoptic 16 and photodetector 18 all operate in a manner known in the art.

As previously discussed in the "Background" section, a photodetector,such as the photodetector 18, is typically a high-speed device thatcannot receive a signal at its input stage carrying more than about 1 to5 mW of optical power before nonlinearities, bandwidth reduction, orcatastrophic damage occurs to the operation of photodetector 18, or thephotodetector 18 itself. Therefore, the presently available high-power(approximately 200 mW) laser sources serving as a carrier wave (CW)source, such as source 12, cannot be utilized by the circuit arrangement10. In addition to carrier wave (CW) source 12 limitations, the usage ofthe electro-optical modulator 14 of FIG. 1 has limitations with regardto its linearity of operation which may be described with reference toFIG. 2.

FIG. 2 has a Y axis, indicated as the optical through put of theelectro-optical modulator 14, and a X axis indicated as being thesinusoidal input signal 22 (V.sub.π (Input Voltage)). For linearityoperation of the electro-optical modulator 14 and for maximumdifferential change in the optical output signal of the electro-opticalmodulator 14 per input volt of the input signal 22, the electro-opticalmodulator 14 is typically biased at a point 34 corresponding to one-halfof the maximum output signal 36. The bias point 34 is called quadratureand is typically accomplished by applying a bias voltage to theelectro-optical modulator 14 in a manner known in the art. When biasedat quadrature (34), the electro-optical modulator 14 optical outputsignal P_(out) may be given by the below expression 1: ##EQU1## whereP_(in) is the input optical power generated by the CW source 12 andapplied to signal path 20 of FIG. 1, k is a constant, x is the inputsignal voltage (signal 22) applied to the electro-optical modulator 14on signal path 24, and the sign (±) of expression (1) depends on theslope of the quadrature point 34 chosen in a manner known in the art.The sinusoidal function of the input signal 22 introduces compression inthe output signal P_(out), unless the modulator depth, m, that is, thedegree to which the carrier wave from the CW source 12 is modulated bythe input signal 22, is kept below about 71%. The modulation depth, m,may be defined by expression 2: ##EQU2## when kx is small the outputsignal (P_(out)) may be approximated by expression 3: ##EQU3## where thepositive sign of expression (3) is taken without loss in generality orapproximation. Under such as assumption, expression (3) may be rewrittenas the below expression 4 which is also generally illustrated in FIG. 3.##EQU4##

FIG. 3 illustrates as its Y axis the P_(out) signal from theelectro-optical modulator 14, as well as illustrates the first componentof the right hand side of expression (4) identified by reference number38, the second component of the right hand side of expression (4)identified by reference number 40, and a band of the component 40identified by reference number 42. The band 42 of component 40 containsthe maximum peaks and valleys of the output signal P_(out), i.e. theportion of the optical carrier 20 modulated by RF signal 24, inelectro-optic modulator such as member. Horizontal lines 42 bracket themaximum and minimum extent of the modulated RF signal onto the carrier,and is the depth of modulation. Line 38 extends up to the minimum of themodulation (bottom line of lines 42), and represents the unmodulatedcarrier, or excess carrier.

Component 40 is the portion of the curve in FIG. 3 disposed betweenhorizontal lines 42, and is the sum of the signal kx, which may bepositive or negative, and is typically just enough of an input opticalcarrier kx_(average) to insure that the modulation depth of thecomponent 40 does not exceed 100%. The component 38 identified asP_(excess) is commonly called the "excess carrier" and represents theundesired unmodulated optical power previously described in the"Background" section having undesired aspects. If this excessivecarrier, that is, component 38 is considered to be a separate opticalcomponent, as expression (4) suggests, the excessive carrier increasesthe signal that the photodetector 18 is subjected to since thephotodetector 18 acts like a coherent high-power local oscillator tofurther increase the power level of the excessive carrier. However, theoutput of the photodetector 18 and its "gain" is linearly proportionalto the excess carrier power (component 38) which is limited by thephotodetector 18 power handling capabilities, that is in the range of 1to 5 mW. The limitation to the photodetector 18, as well as theinability to effectively utilize the excess carrier component 38, inaddition to the incapability of utilizing the existing CW sources 12having a laser output of approximately 200 mW, increase the fiber-opticlink losses that a system employing the circuit arrangement of FIG. 1may experience.

The input electrical power, such as signal 22 of FIG. 1, can beexpressed by expression 5 given below: ##EQU5## where R_(IN) is theinput resistance, and V_(in) peak-peak is the peak-to-peak inputvoltage. The output power from the photodetector 18, that is, signal 30(OUT) of FIG. 1 may be calculated from the peak modulation current,m.I_(PD),_(aver), and may be expressed by expression 6 given below:##EQU6## where R_(LOAD) is the load resistance. The fiber-optic linkloss is the ratio of the output power to the input power and may beexpressed by expression 7 given below: ##EQU7## which reduces to 200 I²_(PD), aver. with typical values of R_(LOAD) =R_(IN) =50Ω and with amodulator V.sub.π of 10 volts (at V_(in) =V.sub.π /4, m=0.71).

Expression (7) indicates that the fiber-optic link loss transmissioncoefficient is proportional to the photodetector 18 average currentsquared. For high-speed photodetectors 18 with an average currentlimited to 1 mA, conventionally the minimum fiber-optic link loss isapproximately 37 dB. So very little output microwave power fromphotodetector 18 is available from signals with small modulation depths.This also results in very little optical power in the modulationsidebands on the optical carrier wave. If a photodetector 18 wasavailable which could detect 70 mA without damage or responsivityreduction, the fiber optic link loss would decrease to 0 dB, resultingin more output power from photodetector 18. However, they do not existyet at microwave frequencies. Therefore, if a technique was available toattenuate the optical carrier wave to create 1 mA photodetector currentsand not attenuate the modulation sidebands, more signal power would beavailable from photodetector 18.

Fiber-optic link loss may also be further considered from a modulationdepth m, analysis. For example, consider an optical signal of 100 μW(-10 dBm average optical power output from the electro-optical modulator14, where dBm is absolute power measured in decibels, and referenced to1 mW). Consider further that this optical signal contains sinusoidaloptimal optical modulation with a 100% modulation depth, m. If theaverage power yielded by electro-optical modulator 14 is increased by afactor of N=10 to 1 mW (0 dBm) by amplification at the output stage ofthe electro-modulator 14, the output signal of the optical-modulator 14detected by the photodetector 18 would be increased by G=100 (20 dBe).That is, assuming a photodetector 18 has a responsivity of 1 A/W and a50Ω load, the original (no amplification) post-detection electricalpower of -36 dBm would be increased to -16 dBm. However, if an opticalsignal of 100 μW (-10 dBm average optical power output from theelectro-optical modulator 14) containing sinusoidal optical modulationof only 1% modulation depth was similarly amplified, the correspondingoriginal (no amplification) post-detection electrical power of -76 dBmwould be increased to -56 dBm. The reduced modulation depth, m, of 1%instead of 100% has reduced the output power of the electro-opticalmodulator 14 by 40 dB. Thus, for these assumptions, a 40 dB improvementin the fiber-optical link loss could be realized if the modulationdepth, m, could be increased to its optimum value of 100%. The presentinvention provides for an apparatus and a method both that increase themodulation depth, m, to keep the fiber-optic link loss to a relativelysmall amount, and, in fact, the output of the electro-optical modulator14, or a similar device, after being conditioned by the presentinvention, that is applied to the photodetector 18 can be reduced to apoint that the overall insertion loss of the fiber-optical link may beeliminated altogether, thereby, decreasing the fiber-optic link loss ofthe system embodying the present invention. The present invention may befirst described with reference to FIG. 4.

FIG. 4 illustrates an arrangement 44 having a Brillouin medium 46, knownin the art and is further described, for example, in the text "ThePrinciples of Nonlinear Optics," of Y. R. Shen published by John Wileyand Sons, New York, 1984 and, also in the text "Quantum Electronics," ofA. Yariv published by John Wiley and Sons, New York, 1989, both of whichare herein incorporated by reference. The Brillouin medium 46 acts as anattenuator and filter for reducing, passing and blocking incidentoptical signals comprising carrier waves having sidebands each having apreselected power level. The Brillouin medium 46 has a threshold forcreating backward traveling Stoke's waves when being subjected to apredetermined power level of incident optical signals. As will befurther described, the threshold of the Brillouin medium 46 is selectedto increase the sideband to carrier wave power ratio so as to increasethe modulation depth, m, of the systems in which the Brillouin medium 46is used, such as wideband filter-optic systems.

The circuit arrangement 44, in addition to the Brillouin medium 46,comprises a carrier wave source 48, a fiber-optic coupler 50 having foursignal paths or ports 50A, 50B, 50C, and 50D, a 50 dB isolator 52, backscattering light 54 generated by the operation of the Brillouin medium46, and a mirror 56 used in the generation of the electrical signal fromthe back scattering light 54 in cooperation with the Brillouin medium46. The carrier wave source 48 generates the optical carrier that isapplied to the fiber-optic coupler 50, via signal path 50A. Thefiber-optic coupler 50 transfers the carrier wave signal to its outputpath 50C that is applied to the Brillouin medium 46. The Brillouinmedium 46 generates an output signal that is applied to the 50 dBisolator 52, via signal path 58 which, in turn, develops an outputsignal 60 (OUT) on signal path 62. The signal path 50C is bidirectionalbecause the Brillouin medium 46 creates backward traveling Stoke's wavesthat are directed back into the fiber-optic coupler 50. The generaloperation of the Brillouin medium 46 may be further described withreference to FIG. 5 having a schematic 64 comprised of directionalarrows or wave vectors, k_(incident), k_(sound) and k_(Stoke's). Therelationship of these wave vectors may be given in the below expression8 where k is a wave vector:

    k.sub.incident +k.sub.sound =k.sub.Stokes                  (8)

The Brillouin medium 46 upon being subjected to an optical signal(k_(incident)) of moderate to high power (to be described) generatessound wave k_(sound). As the power in the optical signal k_(incident)increases the optical wave k_(incident) interacts with the sound wavek_(sound) in the Brillouin medium 46. This interaction causes theincident light k_(incident) to be scattered in accordance to theconservation of energy, sometimes referred to as conservation ofmomentum, thereby, downshifting, that is, decreasing, the frequency ofthe scattered signals. From expression (8) it should be seen thatincident and sound wave vectors are additive to each other to form theStoke's wave vector. The presence of the Stoke's wave (k_(Stoke's))regenerates the sound wave (k_(Sound)) and results in positive feedbackso as to result in a threshold effect, whereby above a given power ofthe incident signal (k_(incident)) the Stoke's waves grow withoutbounds. The Stoke's wave output is limited solely by the depletion ofthe pump, that is, the input signal, such as that supplied from acarrier wave (CW) source 48, having a single-frequency 1550-nm Erbiumlaser with a linewidth less than 100 kHz.

The Brillouin medium 46 of FIG. 4 may comprise a 25 km length ofdispersion shifted single mode optical fiber having a predeterminedstimulated Brillouin scattering (SBS) bandwidth. The SBS bandwidth and atypical input-output-scattered power representation process related tothe Brillouin medium 46 may be further described with reference to FIG.6.

FIG. 6 has an X axis indicating the input optical power in milliwatts(mW), and a Y axis indicating the power output of the Brillouin medium46 given in milliwatts (mW) . The overall operation of the Brillouinmedium 46 is indicated by response 66 which comprises two plots 68 and70 that are respectively representative of the backward traveling power(Stoke's) present on signal path 50C of FIG. 4, and the forwardtraveling power (output) that is present on the signal path 62 of FIG. 4and corresponds to signal 60 (OUT) of FIG. 4.

FIG. 6 further illustrates a point 70 corresponding to a typical SBSthreshold having a typical value of 8 mW, as identified by verticaldimensional line 72. As seen in FIG. 6, the input optical powercorresponding to the SBS threshold, that is, 8 mW represents the point70 that the Stoke's backward traveling (power plot 68) increases rapidly(this actually defines the SBS threshold). The SBS threshold alsoindicates the point 70 that the transmitted power, that is, plot 60, iseffectively clamped. Incident light, that is, the output of the CWsource 48 reaching the Brillouin medium 46 as incident light,k_(incident), above the SBS threshold (8 mW) and within the SBSbandwidth, is scattered backwards. More particularly, incident lightabove the SBS threshold does not get past the Brillouin medium 46, butrather is directed backward to the fiber-optic coupler 50, via signalpath 50C.

The SBS bandwidth is measured by measuring the bandwidth of thescattered light (see backscattering light 54 of FIG. 4). The measuringis accomplished by heterodyning the Stoke's wave, which is developed bythe interaction of the sound wave (k_(sound)) of the Brillouin medium 46with the incident wave (k_(incident)) generated by the carrier wavesource 48, with additional light waves from a mirror 56 that may bedeveloped by a high speed photodetector and applied to the fiber-opticcoupler 50, via signal path 50D. The heterodyning results in a beatnotefrequency centered around the difference frequency between the Stoke'swave and the input signal, for example, the signal supplied by thecarrier wave source 48. The beatnote frequency is dependent upon theBrillouin medium 46, and for the Brillouin medium 46 comprised of fusedsilica, the beatnote frequency is between 10 and 12 GHz. Moreparticularly, for the fused silica Brillouin medium 46 of FIG. 4 havinga length of 25 km, the beatnote frequency is 10.54 GHz and is shown inFIG. 7.

FIG. 7 shows a resulting beatnote frequency signal or heterodyne signal74 where it is seen that the Stoke's shift for the Brillouin medium 46is at the center point 76 corresponding to the frequency of 10.54 GHz.The 3 dB bandwidth of the signal 74 developed by the circuit arrangementof FIG. 4 is less than 10 MHz. The stimulated Brillouin scatteringprovided by the Brillouin medium 46 may also be provided by afiber-optic ring resonator (FORR) which may be described with referenceto FIG. 8.

FIG. 8 illustrates the fiber-optic ring resonator (FORR) comprising afiber-optic coupler 78 having a low coupling ratio. The fiber-optic ringresonator (FORR) 78 recirculates light, indicated by directional arrow80, in a manner known in the art. The FORR device 78 has many usefulapplications, also known in the art, and is more fully described in thetechnical article "Filter Response of Single-Mode Fiber RecirculatingDelay Lines," of J. E. Bowers, S. A. Newton, V. M. Sorin, and H. J.Shaw, published in the Electronic Letters 18, pp. 110-112, 1982, andherein incorporated by reference. The FORR device 78 operates in asimilar manner to that of the Brillouin medium 46 of FIG. 4, but has alower SBS threshold which could be as low as 100 μW. Resonant cavityloop 80 is of Brillouin material, and as light circulates in the loopthe buildup of energy in the cavity will cause Brillouin scattering tooccur at lower input power than would occur were no energy stored in theBrillouin material (e.g., as in embodiments described above). Because ofthis, one would wish the Q of loop cavity 80 to be as high as possibleto maximize energy stored in the loop per unit of input energy to theloop, so as to make the fiber reach its Brillouin threshold at as low aninput power as possible, maximizing filter sensitivity. Statedalternatively, the FORR device 78 doesn't require the same relativelylong length, e.g., 25 km, as that of the Brillouin medium 46 of FIG. 4,to produce the same amount of signal filtering.

It should now be appreciated that the practice of the present inventionprovides for various devices such as the Brillouin medium 46 of FIG. 4or the FORR device 78 of FIG. 8, both of which act as a filter to rejecta particular band of signals having a particular level of power. Each ofthese devices 46 and 78 has a very narrow bandwidth, such as the 10 MHzbandwidth of FIG. 7 for the Brillouin medium 46, and effectively clampthe input power of the incident light, that is, the power of the signalsupplied by the CW source 48 so that light sources producing incidentlight having a power rating approaching 200 mW may be utilized by thepractice of the present invention without causing any detrimentaleffects to a conventional high speed photodetector. One application ofthe present invention may be described with reference to FIG. 9.

FIG. 9 illustrates an arrangement 82, wherein the Brillouin medium 46 ofFIG. 4 is interposed between an optical transmitter 84 and an opticalreceiver 86. The optical transmitter 84 comprises a laser source 88, anelectro-optical modulator 90, a fiber-optic isolator 92, an amplifier 94(340 mW) that may comprise an Erbium-doped fiber amplifier, and afiber-optic isolator 96. The laser source 88 may be a single-frequency1550-nm Erbium laser with a linewidth less than 100 kHz and with a poweroutput of about 50 mW and may be of a type made available by ATX Telecomas their model 1535-EHA. The laser source 88 provides aforward-traveling carrier wave that is applied to the electro-opticalmodulator 90 via signal path 98. The electro-optical modulator 90 may bea Mach-Zehnder modulator, known in the art, that receives an RF signal(RF_(IN)) 100 via signal path 102. The signal 100 (RF) that modulatesthe forward-traveling carrier wave from the source 88 may have a typicalamplitude of 14 volts at a frequency of about 18 GHz. Theelectro-optical modulator 90 modulates the carrier wave generated by thelaser source 88 with the signal 100 (RF_(IN)) and provides an output onsignal path 104 that is routed to the fiber optic isolator 92 which, inturn, provides an output on signal path 106 that is routed to theamplifier 94. The amplifier 94 amplifies and isolates its receivedsignal and generates an output signal on signal path 108 that is appliedto the optical fiber isolator 96 which, in turn, provides an outputsignal comprised of the carrier wave of laser source 88 modulated by theinformation contained in the signal 100 (RF_(IN)) and having sidebands.The fiber-optic isolator 96 delivers its output on signal path 110 thatis routed to the Brillouin medium 46 which, in turn, provides an outputsignal that is applied to the optical fiber isolator 114 of the opticalreceiver 86, via signal path 112. The optical fiber isolator 114 appliesits received signal to a photodetector 116, via signal path 118. Thephotodetector 116 develops an output signal 120 (RF_(OUT)) that isapplied to signal path 122.

In general, the method of operation for the arrangement of FIG. 9 is toprovide a Brillouin medium having a threshold for creating backwardtraveling Stoke's waves when subjected to a predetermined power level ofincident optical signals. Further, the method of operation for thearrangement of FIG. 9 includes selecting the threshold at a power levelso that optical carriers above this threshold are attenuated andsidebands of the optical carriers are allowed to pass. The selectedthreshold establishes a depth of modulation of the information 100(RF_(IN)) that is carried by the optical signals generated by the lasersource 88.

In operation, as the optical transmitted power increases, that is thepower on signal path 110, the Brillouin medium 46 operates, in a manneras described with reference to FIGS. 4-8, so as to scatter all opticalsignals (into their respective Stoke's waves) which are defined by apower level above the SBS threshold of the Brillouin medium 46. TheBrillouin medium 46 scatters signals that have a power level greaterthan, for example, 8 mW and for the laser source 88 this power levelcorresponds to carrier wave signals having a frequency within 25 MHz ofthe carrier wave. Therefore, if the modulation sidebands, contained onthe carrier wave applied to the Brillouin medium 46 are greater than 25MHz of the incident signal (the signal developed by laser source 88) andif the sidebands do not contain sufficient power (8 mW) to activate the(SBS) threshold for the Brillouin medium 46 described with reference toFIG. 6, the sidebands are transmitted without loss of power. That is tosay that if the sidebands of the input signal are spaced from theunmodulated optical carrier by more than the Brillouin linewidth, andthe carrier contains more power than the sidebands, and sufficient powerto drive the Brillouin material into stimulated scattering, the carrierwill trigger stimulated Brillouin scattering at its frequency, and atall frequencies within the Brillouin linewidth. If the sidebands do notcontain sufficient power to drive the Brillouin medium into stimulatedscattering, the sidebands do not experience stimulated Brillouinscattering and pass without added attenuation, thus increasing systemsensitivity. The sidebands and the information they contain are allowedto pass the Brillouin medium 46 without any attenuation thereto and aredirected to the optical receiver 86, in particular, the photodetector116. Furthermore, since the carrier power, that is, the output power ofthe laser source 88 having a typical value of 50 mW, is above the SBSthreshold (8 mW) , the transmitted carrier power, that is, the powermade available on signal path 112, is clamped to the 8 mW quantity (SBSthreshold) . The net effect is to increase the received sideband tocarrier wave power ratio, thereby increasing the modulation depth, m,and also decreasing the fiber-optic link loss of the optical systemembodying the practice of this invention, such as circuit arrangement 82of FIG. 9.

The circuit arrangement 82 is advantageous in that it is self-regulatingbecause no stabling bias control is required. Further, the circuitarrangement 82 is advantageous in that it is self-tuning because thethreshold of the Brillouin medium 46 is at the carrier wavelength,provided the linewidth of the laser source 88 is less than the SBSthreshold bandwidth. The operation of the SBS threshold to reduce thecarrier wave is immune to laser (carrier) drift, provided the drift rateis slower than the order of the SBS threshold bandwidth divided by thepropagation delay, e.g., for FIG. 9, 8 MHz/(25km • 5 μs/km)=64 MHz. Theoperation of the circuit arrangement 82 of FIG. 9 was experimentallyverified and the results of which are shown in FIG. 10.

FIG. 10 illustrates the system 82 responses represented by plots 124,126, 128, and 130 each indicative of particular operational parametersselected for the system 82 and given in Table 1.

                  TABLE 1    ______________________________________    PLOT       SYSTEM OPERATION    ______________________________________    124        JUST BELOW AN SBS THRESHOLD (8               mW INPUT)    126        FOUR (32 mW) TIMES THE SBS               THRESHOLD POWER    128        EIGHT (64 mW) TIMES THE SBS               THRESHOLD POWER    130        EIGHT (64 mW) TIMES THE SBS               THRESHOLD AND WITH THE ELECTRO-               OPTICAL MODULATOR 90 BIAS BELOW               QUADRATURE VIA THE TRANSMISSION               OF APPROXIMATELY 5%    ______________________________________

Table 1 indicates the set of conditions that the Brillouin medium 46 issubject to at its input stage. For example, plot 124 (response shown inFIG. 10) illustrates the response of system 82 when the Brillouin medium46 is subjected to incident optical signals having a power level of justbelow the SBS threshold of 8 mW.

The plots 126 and 128 respectively represent gains of 6 and 8 dB,whereas plot 130 represents an effective gain of 19 dB, as well as astate of operation, and wherein each gain corresponds to the gain in theratio of sideband power relative to the carrier wave power.All the plots124, 126, 128 and 130 represent system responses for the circuitarrangement 82 in which the photodetector 116 of FIG. 8 has the sameoperating current because the transmitted output, that is, the powermade available at signal path 112 is clamped by the operation of theBrillouin medium 46 in a manner as previously described with referenceto FIG. 6. Further, all of the plots 124, 126, 128 and 130 have a gainfactor for the sideband to carrier wave power ratio increases whoseresponse is very broad, such as that between 45 MHz to 20 GHz. The plots124, 126, 128 and 130 could be duplicated by the use of the FORR device78 of FIG. 8 with even still further possible increases in gain factors.

It should now be appreciated that the practice of the present inventionprovides for a stimulated Brillouin scattering effect produced byBrillouin medium that acts as an attenuator and filter passing andrejecting selected frequencies, while clamping or fixing the poweroutput of the Brillouin medium.The selection of the Brillouin mediumallows for the increase of the sideband to carrier wave power ratiowhich, in turn, increases the depth of modulation, m, experienced by theinformation carried by the optical carrier wave.

Further, the practice of the present invention effectively clamps theoutput power of an optical transmitter so as to provide protectionagainst photodetector catastrophic failures or operation degradations ofthe photodetector. Furthermore, the practice of the present inventionprovides for an arrangement that is inherently self-tuning in that theBrillouin medium serving as the filter is always centered on the carrierwavelength having parameters that correspond to the SBS threshold. Thiscentering is maintained so long as the wavelength of the carrier wavedoes not drift rapidly.

Furthermore, the present invention allows for the use of microwave powerfrom microwave optical carrier wave sources having an output power levelapproaching 200 mW.However, if desired, a FORR device 78 may be utilizedand still provide for a SBS power threshold of a relatively low value sothat the practice of the present invention may be utilized for opticalcarrier wave sources having a power rating of 1-5 mW.

Although the hereinbefore given description is related to a Brillouinmedium comprised of silica or the FORR device both having the stimulatedBrillouin scattering effect, other Brillouin mediums, known in the art,may be used to accomplish the same stimulated Brillouin scatteringeffect.

It should, therefore, readily be understood that many modifications andvariations of the present invention are possible within the purview ofthe claimed invention.It is therefore to be understood that, within thescope of the appended claims, the invention may be practiced otherwisethan as specifically described.

What I claim is:
 1. An attenuator and filter for reducing, passing andblocking incident optical signals comprising carrier waves havingsidebands each having a preselected power level, said attenuator andfilter comprising a Brillouin medium having a threshold for creatingbackward traveling Stoke's waves when subjected to a predetermined powerlevel of incident optical signal, said threshold being selected toincrease the sideband to carrier wave power ratio.
 2. The attenuator andfilter according to claim 1, wherein said predetermined power level isin excess of about 5 milliwatts (mW).
 3. The attenuator and filteraccording to claim 1, wherein said incident optical signal is generatedby a laser source having an operating wavelength of about 1550 nm with alinewidth less than about 100 kHz.
 4. The attenuator and filteraccording to claim 3, wherein said Brillouin medium is a dispersionshifted single mode optical fiber comprised of fused silica and has alength of about 25 km.
 5. The attenuator and filter according to claim3, wherein said Brillouin medium is a fiber-optic ring resonator.
 6. Amethod of increasing the power ratio of sidebands to their incidentoptical carrier waves comprising the steps of:(a) providing a Brillouinmedium having a threshold for creating backward traveling Stoke's waveswhen subjected to a predetermined power level of incident opticalsignals; and (b) selecting said threshold at a power level so thatoptical carrier signals above this threshold are attenuated and thesidebands of said optical carrier waves are allowed to pass.
 7. Themethod of claim 6, wherein said selecting step further comprisingselecting the threshold to establish a depth of modulation of theinformation carried by said incident optical signals.
 8. The method ofclaim 6, wherein said predetermined power level has a value greater thanabout 5 milliwatts (mW) but less than 50 milliwatts (mW).
 9. The methodof claim 6, wherein said predetermined power level has a value of about10 milliwatts.
 10. The method according to claim 6, wherein saidBrillouin medium is a dispersion shifted single mode optical fibercomprised of fused silica and having a length of about 25 km.
 11. Themethod according to claim 6, wherein said Brillouin medium is afiber-optic ring resonator.
 12. An attenuator and filtering system forreducing, passing and blocking optical signals generated by an opticaltransmitter and received by an optical receiver, said opticaltransmitter having a source providing optical carrier waves in excess ofabout 5 milliwatts (mW) with sidebands thereof and an electro-opticalmodulator, said optical receiver having a photodetector responsive toreceived optical carrier waves and said sidebands thereof, saidattenuator and filtering system being interposed between said opticaltransmitter and said optical receiver and comprising a Brillouin mediumhaving a threshold for creating backward traveling Stoke's waves whensubjected to a predetermined power level in excess of about 5 milliwatts(mW), said threshold being selected to establish a predetermined ratioof said sidebands and said carrier waves.
 13. The attenuator andfiltering system according to claim 12, wherein said threshold isfurther selected to determine the depth of modulation of the informationcarried by said carrier wave and said sidebands and received by saidphotodetector.